The expression of flag=false if (flag) {return} timer=null if (timer) {return} in the throttle valve has been unclear

I don't quite understand this if(flag) if(timer) What it stands for

flag=false 
 if(flag){return}  

 timer=null   
 if(timer){return}

flag=false if(flag){return} // If flag by ture Just return Exit function

sentence , Only in if() Medium boolean by true when , To perform if() The following sentence {},
If false when , perform else below {};

timer=null if(timer){return}// If timer There are values by true sign out Function? ？

null Namely Basic types of data null class , Chinese means empty ,
When used null As a condition of judgment , Will default back to false,
This is a js A hidden function of .

stay if( No matter what kind of statement, it must == true) To perform if Below {} The statement in , If equal to false when , Execution is else Below {} The statement in .